Gabon opposition leader Jean Ping has been prevented from taking a flight to France by security forces who said he needed to stay over a judicial matter, he said.

Ping, a 75-year-old career diplomat, was narrowly defeated by incumbent Ali Bongo in last year's presidential election, a victory that he claims was tainted by electoral fraud.

"I was supposed to travel this evening but the security forces prevented me," the former African Union chief told AFP.

Ping said he had not been notified of any ban on leaving the country, but said it appeared to be linked to a judicial request that he help with an inquiry in which he has been called as a witness.

An official-looking letter banning him from leaving the country, on grounds he needed to be available to the judge leading an inquiry into a protester arrested in September, was widely circulated on social media but AFP was unable to verify its authenticity.

"The courts are on strike," he said, referring to a strike led by magistrates since mid-December. "How could they issue such a document so quickly?"

Last night in Paris, about 30 of Ping's supporters occupied a mansion in the city's upscale Seventh Arrondissement belonging to Bongo, which they say was purchased under dubious circumstances.

The protesters said they were denouncing the government's move to keep Ping from coming to France, as well as "embezzlement" of state funds by the Bongo family.

"President Bongo spent a fortune, more than 200 million euros, to buy and renovate this mansion," one of the protesters, Jean-Felix Adande, told AFP.

"This is wrongly acquired property, which belongs to the Gabonese people, so the Gabonese people are taking it back," he said.

Another demonstrator, Doriane Ozenga, slammed the "shocking luxury" of the mansion, "with its immense rooms and gold-plated doorknobs."

The mansion is owned by the president of a country where about a third of its 1.8 million people live below the poverty line.

The protesters stayed for several hours at the Pozzo di Borgo mansion, which has been empty during renovation works.

French anti-corruption groups have accused Bongo and his family of siphoning off Gabon's wealth, but an inquiry led by French judges was closed in August last year, with no charges filed.

Ping had faced a similar ban on leaving Gabon in September after the government accused him of making "inflammatory comments", but it was lifted about a week later.

Gabon's parliament on Wednesday approved a raft of changes to the constitution which was denounced by the opposition as a power grab, with the proposal now set to be approved by the constitutional court.

Where the previous version said the president would rule "in consultation with the government", the new text allows him to "determine the policy of the nation" while also ruling out any limit on the number of times he can be re-elected.

Bongo has ruled Gabon since 2009, taking control after the death of his father, Omar Bongo, who had been in power since 1967.

He said last week that the new constitution paved the way for long-delayed legislative elections, "demonstrating the vitality of our democracy and the sealing of the political cohesion of our nation."

The elections were supposed to have taken place in December 2016 but have been postponed twice. The latest date given for them is April this year.

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